This invention relates to actuators, and more particularly to the design of a linear electromagnetic zero net mass jet (ZNMJ) actuator for providing improved output velocity of jet air pulses therefrom.
All modern military and commercial aircraft/rotorcraft include aerodynamic lifting surfaces that can significantly impact the performance of the entire vehicle. In general, the aerodynamic efficiency of any lifting surface, regardless of the class of vehicle, is dependent on the lift-to-drag ratio of that surface. Recent wind tunnel tests conducted at the Boeing Company have demonstrated that significant improvements in this ratio (through increases in lift and reductions in drag) can be achieved with the use of a zero net mass jet (ZNMJ) actuator. Specifically, the tests demonstrated that the percent improvement is directly related to the momentum output of the ZNMJ actuator and its capability to perform in conditions representative of those encountered in realistic flight situations. Consequently, a high-momentum output ZNMJ actuator having an expanded performance envelope is one that will guarantee additional aerodynamic Improvements. Such aerodynamic improvements involve improving the range of commercial transports for a given amount of fuel, increasing the maneuverability of fighter aircraft and increasing the lifting capability of heavy lift aircraft/rotorcraft.
The recent wind tunnel experiments described above are supported by results from numerical simulations that indicate that the aerodynamic benefits resulting from the use of an electromagnetic ZNMJ actuator are directly proportional to the momentum output of the actuator (or equivalently, to the actuator""s maximum output velocity). These benefits, be they stall/post-stall, lift enhancement and/or drag reduction, have been found to be a result of preventing/suppressing separation of the boundary layer due to periodic injection of momentum into the flow.
In general, current ZNMJ actuator designs suffer from performance degradation manifested by low-momentum output when such actuators are operated at high frequencies (200 Hz-600 Hz) and/or, for a given frequency, when operated at a safe input voltage. These two limitations restrict the class of practical problems that can benefit from the application of active flow control (AFC). For example, for an aircraft flying at low to high subsonic free stream Mach numbers (e.g. 0.20-0.50), enhancement of the aerodynamic lift on the wing, and/or the reduction of airframe drag, are known to demand high momentum output if measurable aerodynamic benefits are to be expected. Unfortunately, to date, these benefits have not been realized due to the performance limitations of present day ZNMJ actuators. The ideal ZNMJ actuator would provide consistent performance (i.e., momentum output) over a wide range of operating voltages, frequencies and actuator geometric parameters.
Referring to FIG. 1, a typical prior art ZNMJ actuator is shown. As perceived by an observer standing next to the orifice-like port 1, periodic flow out of and into the cavity 2 is seen. The external jet-like flow 3 results from the entrainment of the surrounding ambient fluid 4. Typically, the diaphragm (or piston) of a ZNMJ actuator 5 is activated electrostatically, electromagnetically, or through the use of a piezoelectric material with frequencies that span 1-10 KHz. In short, the mechanics of the jet resemble those associated with the outward and inward flows observed when one moves a piston forward (i.e., towards the orifice) and backward (i.e., away from the orifice) in a cylinder having a single exit port. An electromagnetic ZNMJ actuator functions in a very similar way. Here the motion of the piston is emulated with a rigid diaphragm that oscillates 6 inside a cavity 2 by means of an electromagnet to duplicate the repeated forward and backward motions of the piston inside the cylinder 7.
The performance of a typical ZNMJ actuator is characterized by the measured jet 3 momentum (or equivalently, the maximum external velocity) a small distance away from the orifice-like exit port 1. With no exception, for a given voltage, all ZNMJ actuator designs experience rapid degradation in momentum output at high oscillation frequencies. For full scale aerospace vehicles, effective flow control for providing measurable aerodynamic benefits, be it increasing the lift of an airfoil/wing/rotor or reducing the drag of an airfoil/wing/rotor/fuselage/airframe, at realistic flight Reynolds numbers and free stream Mach numbers, demand higher momentum levels at low to moderate frequencies (e.g., 50-200 Hz).
It will also be appreciated that the momentum output of a ZNMJ actuator is a function of the applied voltage to the actuator. In principal, the actuator can achieve higher momentum (or output velocities) through higher imposed voltages. However, since the actuator is an electrical device, its safe operation is dictated by the maximum allowable power which, indirectly, translates into a xe2x80x9csafexe2x80x9d operating voltage and/or a xe2x80x9csafexe2x80x9d operating current. Typically, this operating voltage is 10-15% lower than that based on the maximum power requirement.
In view of the foregoing, it is a principal object of the present invention to provide a new ZNMJ actuator design which can provide a higher momentum (i.e., velocity) output for a given input voltage. It is a further object of the present invention to provide an electromagnetic ZNMJ actuator which functions to provide an even higher momentum output over a given frequency range than previously developed ZNMJ actuators, and which does so without significantly increasing the weight and volume of the actuator, and without complicating the construction of the actuator.
The present invention is directed to a zero net mass jet (ZNMJ) actuator in accordance with preferred embodiments of the present invention. In one preferred embodiment, the actuator comprises a linear electromagnetic ZNMJ actuator. The actuator includes a first permanent magnet and a second permanent magnet disposed closely adjacent the first permanent magnet so as to define an air gap therebetween. A coil is disposed in the air gap. An air moving member is connected to the coil. The moving member acts as a piston, and in one preferred form comprises a diaphragm. The coil experiences a concentrated magnetic flux field due to the arrangement of the poles of the two magnets. Accordingly, when an alternating current is applied to the coil, this causes the coil, and therefore the air moving member, to oscillate, thereby creating jets of air flow having an increased velocity (i.e., momentum output) over an otherwise conventional ZNMJ actuator incorporating only a single permanent magnet.
In preferred embodiments the ZNMJ actuator of the present invention further includes a keeper for supporting the two permanent magnets. The two permanent magnets are preferably rare earth magnets. In one embodiment, the first permanent magnet comprises an annular shape and the second permanent magnet being is disposed concentrically within an opening of the first permanent magnet. The poles of the two magnets are further arranged such that opposite poles are disposed on opposite sides of the air gap. This serves to minimize the leakage of the magnetic flux due to the forced parallel alignment of the magnetic moments across the air gap, thus increasing the density of the magnetic flux lines intercepted by the coil.
The actuator of the present invention, through its dual magnet configuration, provides increased momentum output for any given input voltage to its coil. The increased momentum output is further provided over a broad frequency spectrum. As additional benefits, the overall size and weight of the actuator are not significantly increased over conventional single magnet ZNMJ actuators.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.